Make Cluster Manager correctly recognize newly added nodes

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Helix based cluster manager does not dynamically recognize newly added nodes. It assumes that `InstanceConfig` change notifications come in only for nodes that it already knows of. When such notifications come in for newly added nodes, the cluster manager meets with a NPE, because it tries to dereference the return value from the hash map of existing nodes (which it expects to always return a non-null value).

At a high level, the way to handle this within `onInstanceConfigNotification()` is to look up the instance in question in the hash map, and if it is absent, then add it as a new entry in much the same way as instance additions are done during initialization.

Start by locating the Cluster Manager implementation and its onInstanceConfigNotification() handler, then compare its handling of known instances with instance additions during initialization. Reproduce the notification for a newly added node and verify that the node is added without a null dereference and is subsequently recognized by the manager.

Written by the indexing model from the issue text.